The most popular types from backyard baseball to pro baseball. They use a spinning wheel(s) & a motor to propel the wheel(s). Most wheel pitching machines have adjustable speed controls & direction adjustments mounted on a steel frame that's attached to a tripod. All run on 110v current & can handle a generator. Most can be purchased as baseball pitching machines, softball pitching machines & some are in a combo unit for both sports. 1 wheel is entry level, 2 wheel are most popular & now 3 & 4 wheels that throw about anything you could want at Major League speeds.

Arm Style Pitching Machines - 4 rack fed & hopper fed models

These are sometimes known as Iron Mike Pitching Machines. That is the made in USA brand you may have seen & used at a commercial batting cage. They weatherproof & tough enough for years of service without trouble. All have auto ball feeders in a rack or a huge hopper that holds hundreds of balls.